This invention is concerned with anti-lock-controlled brake systems in which a pump used to supply fluid under pressure during a control cycle is also connected to the master cylinder. Such a brake system is known from German Published Patent Application (DE-OS) No. 36 27 000. The system is characterized in that the pumps, delivering fluid into the brake circuits during a control operation, directly communicate with the working chambers of the master brake cylinder. The working pistons defining the working chambers are equipped with so-called central valves working as pressure control valves, that operate in such a manner as to adjust the pump pressure to be proportional to the pressure generated in the master cylinder by the brake pedal. The elements of the central valves, i.e., the valve body and the valve seat, need to be of a hardened material such as of metal in order to prevent damage during the opening and closing operations, which take place in uninterrupted succession during a control operation. Further, the pistons are provided with sealing cups acting as a non-return valve so as to ensure that, in case of a quick release of the brake, fluid can enter the brake circuits from the storage reservoir via the sealing cups.
There are also known conventional master brake cylinders provided with so-called compensating holes. Directly in front of a sealing cup, these holes lead into the working chamber of the master brake cylinder from the reservoir. With the master brake cylinder not being actuated, communication exists between the brake circuits and the storage reservoir via the compensating hole. Upon actuation of the master brake cylinder, the working pistons are displaced and the sealing cup slides across the compensating hole so that the brake circuits will be locked hydraulically and a pressure build-up will be possible. Such master brake cylinders cannot be used in anti-lock-controlled brake systems of the above-mentioned type since, with a control of the pump pressure proportional to the brake pedal pressure, the sealing cups would move past the compensating holes under a high pressure and, in doing so, would be destroyed.
In principle, it would be possible to manufacture the sealing cup of a harder material so that there would be no damage during control cycles. In this case, however, the seal would be unable to perform a nonreturn valve function as does a soft material sealing cup.
For this reason, master cylinders with central valves in accordance with the above-mentioned prior art were used in anti-lock-controlled brake systems. It goes without saying, that the manufacture of such master brake cylinders with central valves is considerably more expensive and intricate than that of master brake cylinders for conventional brake systems.
It is thus an object of this invention to design an anti-lock-controlled brake system such as to enable the use of a simple master brake cylinder.